Present day high power laser sources exhibit high quality beams with very high brilliance in the multi-kW power range. For instance, commercial fiber lasers are available with single-mode power of 1 kW and above. Using such high brilliance laser sources in practical applications offers various benefits compared to older generation sources with lower beam quality, such as larger focusing depth and smaller spot size.
However, high brilliance lasers may also give rise to new problems that are not encountered with sources having lower brilliance. A particular problem arises in optics, such as beam collimating and focusing lenses that are used at the output of the laser source to condition the beam for a practical application. The phenomenon behind this problem is called thermal lensing. Physically thermal lensing is caused by spatial and power density dependent variation of the refractive index of the glass material used to construct lenses and other optical elements. Thermal lensing may cause distortions to the output beam, such as power dependent spot size and focal depth variations. Such distortions sometimes make it difficult to control the particular application that the laser is being used for.
One practical example of problems relating to thermal lensing relates to upgrading existing laser sources with new ones. As a manufacturer that uses the laser for an industrial application may exchange an older generation laser source to a higher brilliance new generation laser source, thermal lensing may become a serious issue. Sometimes changing the output optics to better adapt to the laser source is not a viable option due to technical or economical reasons. On the other hand, a laser manufacturer may not want to change the manufacturing chain for the high power laser source to adapt to customer's need for a lower brilliance source.
It is known that optical modes of an optical fiber may be mixed, or to express it more accurately, optical power carried by the optical modes may be redistributed among the modes when a perturbation to the generally cylindrical symmetry of the fiber is produced. A perturbation may be such that the refractive index of the fiber is perturbed, while the geometry of the fiber is preserved. An example of such a perturbation is distributed Bragg reflector that causes mode coupling generally between counter-propagating modes. A localized external pressure may also couple modes with each other through the induced refractive index variation. Fiber bends achieved by e.g. coiling the fiber around a mandrel have been used to produce variation in the effective refractive index of the fiber. However, an efficient mode mixing in such geometries usually require tight coiling radii and/or long lengths of fiber to be used. It may also be that the coiling radius requirement to achieve efficient mode mixing is not practical due to mechanical strength limits of the fiber. Variations to the fiber geometry may also cause mode mixing. Examples of such variations include bulges or other longitudinal translational symmetry breaking features on the fiber. However, the amount of mode mixing by such features is difficult to control.
For the above reasons, among others, there is a need for effective new ways of reducing the brilliance of laser sources in a controlled way to match various customer needs. In particular, such solutions would be needed which are not limited by the mechanical strength properties of the fiber and whose production can be well controlled.
Without directly addressing the present problem in full, JP 59037503 discloses an optical mode scrambler comprising a fiber with three successive biconical taper parts. The biconical parts cause mode conversion and coupling by introducing sections where the core diameter decreases and increases gradually, resulting in a high-performance mode scrambler. A manufacturing method involving pulling of a fiber in heated state is also described. A problem in this kind of design and manufacturing method is that the degree of mode scrambling cannot be accurately controlled due to manufacturing tolerances.
Thus, a problem still remains relating to the controllability and accurate manufacturing of the mode scrambling components.